Early French and German Signals

The disque rouge and the Austrian bell signals had many descendants

In a French book on telegraphs translated into English in 1895, brief mention is made of the application to railway signalling in France. The description seems remarkably archaic for this date, but probably reflects practice in the 1870's or even earlier.

The author describes movement on double tracks in which trains are protected from collision by the display of a red flag by the guard, who goes back from a stopped train to protect its rear from following traffic. This method was quite satisfactory when speeds were low and traffic was light, and effective brakes (which did not exist then) make it even more practical. In many cases, however, it was inconvenient, as when the train was stopped for traffic reasons at a station or junction, and intended to proceed promptly. A solution for this was found in the use of signals remotely controlled by wires. Such signals could be operated by men at the station or junction, who were in a position to know when the way was clear, and when a train should be warned to approach with caution.

In Britain, such a signal, before semaphores became universal, often took the form of a disc or target rotating about a vertical axis and operated by a wire and counterweights. This suggested the French disque rouge, which was placed 500 to 800 metres in rear of the point of restriction. This signal was invented by Robert of the Chemin de Fer du Nord, who also invented the red-and-white chequerboard carré, in the 1860's. When the disc was parallel to the track, showing its edge to the driver, the train could proceed normally. When it was turned face to the driver, the train was instructed to reduce speed immediately, and to approach the point of danger ahead prepared to stop short of any absolute stop signal, obstruction, crossing or junction ahead. Such a signal is often criticised for not showing a positive clear signal, but, of course, it actually does. It is just that the clear signal is not as distinctly given. Drivers no doubt knew where each one was, and recognized when it was at clear. The reference does not state the colour of the disc, but the light at night was red, and the disc itself was without doubt painted this colour, with a white border. The reverse was white with a black border. Yellow, or some other colour, could well have been used in related signals with a different purpose (yellow discs were stop signals on auxiliary lines). The red disc was still used exactly as described in Spain until quite recently. Sometimes it did not rotate, but was permanently fixed at danger. The disc was usually 1000 mm in diameter, and was about 4000 mm high (these are only typical dimensions).

The figure clearly shows how the signal was operated. The handle at the right has a weight that holds it horizontal, as shown, and in this position the disc is parallel to the track. When it is raised, the weight at the other end descends, and eventually overpowers the first weight. If the wire should break, the disc automatically is rotated perpendicular to the track. This causes the disc to be held in the position perpendicular to the track. A lamp is also shown, as taken down for filling, trimming, and lighting. It is hoisted up on its track until it shines through the red roundel in the disc. The figure in the book appears as shown. However, I think the lamp is on a separate support behind the disc, and the whole signal should actually be rotated a quarter-turn. In the earliest signals, the lamp was fixed to the disc and rotated with it. Another lens showed the white light. What we see in the figure is what the driver sees approaching a signal at danger. When the disc rotates parallel to the track, the light shines alone, giving a white light for clear. This is shown in the figure below.

A switch is attached to the vertical axis that closes a circuit when the disc is perpendicular to the line. The wire returns to the point from which the signal is operated, and rings a trembling bell there. The ringing of the bell gives assurance that the disc has responded to the wire, especially when the signal cannot be seen from the point of operation. There was the suggestion that this circuit include a thermoelectric element so that the bell would ring only if the lamp were burning, but it is said that this was not done. The electrical circuit and bell was a more reliable alternative to a wire that was operated by the signal, operating a miniature repeater at the point of control, which gave the same assurance. Some repeaters were not bells, but showed a small red disc in a window. Such electrical repeaters, as well as thermoelectric lamp detectors, were very commonly used in mechanical signalling everywhere.

In Belgium, the red disc was used as an absolute stop signal, while the analogue to the French disque rouge was a rectangular red target. Belgian rules originally required a stop at the advanced signal, after which a train could advance with care. Later, a driver was only required to bring his train under control as soon as possible on sighting a closed disc. A train was not considered protected until it had advanced by the stopping distance, generally taken to be 800 m. The maximum speed at those times was about 100 km/h.

The diagram at the right shows how a small station was protected by disques rouges. 800 m in advance of the disques was a poteau de protection, showing where a standing train would be safe. These posts were placed, for example, 400 m from the point to be protected, whether the end of the platform (quai) or the crossing. Note that the points are trailing from both directions. A train could clear the main line by backing into the service track, a movement called refoulement.

For operating single lines, the author says the French railways have adopted the German bell signals, which really originated in Austria-Hungary. The bells are actually gongs, struck by an electrically released mechanism using a descending weight for power. The circuit is normally closed. Breaking the circuit gives one loud sound. There is one sound every time the circuit is broken and re-established, and a code of signals is based on this. There are codes for (1) up train departed; (2) down train departed; (3) send help; (4) stop all trains immediately; and (5) runaway vehicles on the line. A circuit extended between two stations, with bells at the corresponding ends of the two stations, as well at level crossings and other intermediate points. A station gave the code 'train departed' on the gong, and then sent the train ahead. The station at the other end would wait until that train arrived before sending one in the opposite direction. Apparently, trains could follow one another under this system, since no code for 'train arrived' was mentioned. This information was probably sent by the auxiliary telegraph or telephone. For more details, see Electric Bell Signals.